Chapter 9 449
IF Section
Log Amplifier (P/O A4 Assembly)
2. If an FM signal cannot be demodulated, perform the Demodulator
Adjustment procedure. If the output of A4C707 cannot be adjusted
as described in the Demodulator Adjustment procedure,
troubleshoot the FM Demodulator or Audio MUX circuits on A4.
4.8 kHz IF Filters
Refer to function block N of A4 Log Amplifier Schematic Diagram (sheet
3 of 4) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
Problems with the 4.8 kHz filters can result in spurious signals
appearing 2.88 kHz to 3.52 kHz greater than the frequency of the
desired response. Also, ERR 536 RBW <300 may occur when problems
exist with the 4.8 kHz IF filters.
Measure the passband of the 4.8 kHz IF filters as described in the
following procedure:
1.Onthe8564ECor8565ECspectrumanalyzer,press
CAL,IFADJ
OFF, SPAN, and 600 Hz.
2. Disconnect W27 from A4J3 and inject a 10.7 MHz signal of −20 dBm
into A4J3.
3. Fine-tune the frequency of the signal generator to center the signal
on the screen. Set the signal generator to sweep one 2 kHz span
about this center frequency. Press
SGL SWP on the 8564EC or
8565EC spectrum analyzer.
4. Set another spectrum analyzer, such as the 8566A/B, to 4.8 kHz
center frequency and 2 kHz span.
CAUTION If a dc block is not used, damage to the 8566A/B results. The 8566A/B
and many other spectrum analyzers have dc-coupled inputs and cannot
tolerate dc voltages on their inputs.
5.Connect the VIDEO OUTPUT (rear panel) of the 8564EC or
8565EC spectrum analyzer through a 20 dB attenuator and dc
block to the input of the 8566A/B. Set the sweep time of the 8566A/B
to 10 seconds.
6. Set the 8566A/B to single trigger and press TRACE A
CLEAR-WRITE.
Trigger a sweep of the 8566A/B and the signal generator
simultaneously. The 8566A/B shows the passband of the 4.8 kHz IF
filters. The 3 dB bandwidth of the filters should be 1.2 kHz. The
passband of the filters should be flat within 2 dB over 800 Hz.
7. Reconnect W27 (coax 3) to A4J3.
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